We have isolated a novel liver-specific organic anion transporter, LST-1, that is expressed exclusively in the human, rat, and mouse liver. LST-1 is a new gene family located between the organic anion transporter family and prostaglandin transporter. LST-1 transports taurocholate (K m ‫؍‬ 13.6 M) in a sodium-independent manner. LST-1 also shows broad substrate specificity. It transports conjugated steroids (dehydroepiandrosterone sulfate, estradiol-17␤-glucuronide, and estrone-3-sulfate), eicosanoids (prostaglandin E 2 , thromboxane B 2 , leukotriene C 4 , leukotriene E 4 ), and thyroid hormones (thyroxine, K m ‫؍‬ 3.0 M and triiodothyronine, K m ‫؍‬ 2.7 M), reflecting hepatic multispecificity.LST-1 is probably the most important transporter in human liver for clearance of bile acids and organic anions because hepatic levels of another organic anion transporter, OATP, is very low. This is also the first report of the human molecule that transports thyroid hormones.One of the major function of the liver is the removal of various endogenous and exogenous compounds from the circulation (1, 2). This clearance process involves basolateral membrane transport systems that mediate the hepatocellular uptake of bile acids, organic anions, and organic cations (3, 4). One well studied class of substrates are the bile acids. The uptake of taurocholate is mainly mediated by the Na ϩ /taurocholate cotransporting polypeptide (ntcp) in a Na ϩ -dependent manner (5). The uptake of other bile acids (e.g. cholate) occurs predominantly via a Na ϩ -independent mechanism (2, 4). Some amount of taurocholate is also transported by the Na ϩ -independent mechanism. This Na ϩ -independent carrier system further shows a broad substrate specificity transporting conjugated steroids, cardiac glycosides, and other xenobiotics (4).Initially, the organic anion transporter (oatp) 1 family (oatp1, oatp2, oatp3) was considered to represent the Na ϩ -independent transporting mechanisms in the liver (6 -8). Subsequently, a human cDNA, termed OATP, was isolated (9). However, significant differences were found between human OATP and rat oatp family. First, although the substrate specificities were qualitatively similar, significant differences were found between human OATP-and rat oatp family-mediated initial uptake rates and apparent K m values (10, 11). Second, Northern blot analysis of the human OATP showed considerably high expression in the brain, a pattern that is different from any of the oatp family members. These findings strongly suggest the existence of a different group of organic anion transporters in human liver.Here we report the isolation of a novel human organic anion transporter, termed LST-1, which is expressed exclusively in the liver. When expressed in Xenopus oocytes, many of the functional characteristics of LST-1 were identical to the multispecific transporting mechanisms of human liver. These results suggest that LST-1 is the predominant clearance mechanism of several endogenous and exogenous substrates in human liver. MATERIALS ...

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The rabbit polyclonal antibody against rat organic anion transporting polypeptide 2 (oatp2) was raised and immunoaffinity-purified. Western blot analysis for oatp2 detected two bands ( 74 and 76 kDa) in rat brain and a single band (76 kDa) in the liver. By immunohistochemical analysis, the oatp2 immunoreactivity was specifically high at the basolateral membrane of rat hepatocytes. Functionally, the oatp2-expressing oocytes were found to transport dehydroepiandrosterone sulfate, delta1 opioid receptor agonist [D-Pen2,D-Pen5]enkephalin, Leuenkephalin, and biotin significantly, as well as the substrates previously reported. These data reveal the exact distribution of the rat oatp2 at the protein level in the liver, and that oatp2 appears to be involved in the multispecificity of the uptaking substrates in the liver and brain.

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Antisense oligonucleotides (AONs) that specifically target the genes of rat organic anion transporting polypeptide (oatp) subtypes were selected by using antisense in vitro selection (AIVS) and a conventional gene alignment program (GAP). When we incorporated several of our original 2'-O,4'-C-ethylene-bridged nucleic acid (ENA) residues into AONs, which were designed as gapmers containing a series of 2'-deoxynucleotides in the center, at both the 3' and 5' ends, the inhibitory activity of these oatp AONs was enhanced and their inhibition was mediated by RNase H cleavage. Moreover, these ENA AONs did not lose their oatp selectivity. These strategies of using AIVS and GAP to select AONs followed by incorporation of ENA residues were effective for synthesizing oatp subtype-specific AONs.

To gain a better understanding of the metabolic stability and transport of pitavastatin (CAS 147526-32-7), a new and potent 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, experiments were conducted using human hepatocytes and oocytes of Xenopus laevis expressing human organic anion transporting polypeptide-2 (OATP2), respectively. Almost the entire radioactivity was from the unchanged substance or lactone form in human hepatocytes, and the cytochrome P450 (CYP)-mediated metabolism of pitavastatin was negligible. The results suggested that CYPs are not critically involved in determining the metabolic fate of pitavastatin. The hepatic uptake of pitavastatin reached saturation with a Km of 2.99 +/- 0.79 micromol/L. Also, the uptake of pitavastatin was mediated by OATP2 expressed in oocytes with a Km of 5.53 +/- 1.70 micromol/L. These results indicated that OATP2 plays a major role in the distribution of pitavastatin in liver. Furthermore, to elucidate the increase in the plasma concentration of pitavastatin in a clinical setting, the inhibitory effect of ciclosporin (cyclosporin A, CAS 59865-13-3) on the uptake of pitavastatin was examined. The uptake of pitavastatin was inhibited in the presence of cyclosporin A and the apparent IC50 value was 2.91 +/- 0.78 micromol/L. This result may at least partly explain the drug-drug interaction between pitavastatin and cyclosporin A. In conclusion, the characterization of transporters needs to be taken into account to avoid transporter-mediated drug-drug interaction.